You should try changed your prenstal vitamins, sometimes that's the case too much iron causes constipation.
During my first tri I went through that.... I drank a small cup of prune juice... It was the only thing that helped me
I take fiber laxatives to keep my bowels moving. I have a lot of pressure down there so it hurts if I have to do any extra pushing. Make sure also ur drinking lots of water and fiber enriched foods. Bran cereals, fruits, veggies, etc. I can tell when I haven't had enough fiber.